In a machine tool which has a linear feed axis and a rotational feed axis, it is generally difficult to position a tool at a desired position since an error may occur when the feed axes are moved in accordance with a movement command. Therefore, when highly precise processing is desired, correction has to be carried out depending on the machine error. In order to carry out the correction, the machine error needs to be measured accurately as a preliminary step of correction. The following documents disclose the prior art of measurement and correction of errors.
In Japanese Patent Publication of Examined Patent Application (Kokoku Publication) No. H06-088192 (JP-H06-088192 B1), a method is disclosed in which deviations of two rotational feed axes (deviation of the position of the center of axes) of a machine tool which has two rotational feed axes (A, B) that are orthogonal to each other are measured in advance, and coordinates of the two rotational feed axes are determined by taking these deviations into account.
In Japanese Patent Publication of Unexamined Patent Application (Kokai Publication) No. 2004-272887 (JP-2004-272887-A1), a method is disclosed in which, in a machine tool which has three linear feed axes (X, Y, Z) that are orthogonal to each other and two rotational feed axes (A, C) that are orthogonal to each other, the position to which the machine has to be actually moved is determined based on deviations of the center of a rotating shaft or the center of turning of a spindle, and by using drive control means, the linear feed axes and the rotational feed axes are moved to the determined position so as to correct the position of the tool tip.
In Japanese Patent Publication of Unexamined Patent Application (Kokai Publication) No. H09-237112 (JP-09-237112-A1), a method is disclosed in which an error of a tool unit of a machine tool having a parallel link mechanism is corrected based on an error map. The error map has error data corresponding to each lattice point that have been calculated by computation from the difference between the command value of a position and a posture for tool tip of the tool unit and the measured value of them.
In the pamphlet of WO 2004/034164, a system and a process for measuring, correcting and testing numerically controlled machine tool heads and/or tables are disclosed which are automated and integrated in a numerically controlled system. This system comprises at least one support base that is equipped with a plurality of distance sensors, and at least one device of gauge tool type that is equipped with connection means for the heads at one of its ends and with a ball at another opposite end. The ball is placed next to the distance sensors, so that the distance sensors are able, at any time and in any position, to move to any position in order to measure the distance that separates them from the ball, and to thereby determine the position in the Cartesian space.
In the correction methods disclosed in JP-H06-088192 B1 and JP-2004-272887 A1, deviation of the rotational axis is corrected. Thus, there is a problem that errors or the like which vary depending on the undulation of the rotational axis itself or on the position of the linear feed axis cannot be corrected. The error map disclosed in JP-H09-237112 A1 is the error of the tip of a tool unit that is driven by a parallel link mechanism obtained as table data. Thus, there is a problem that this method is not applicable to a machine tool which has a linear feed axis and a rotational feed axis. In the measurement method disclosed in the pamphlet of WO 2004/034164, only the center position of a reference ball is measured, so that there is a problem that deviation of the position of the tool tip produced due to an error of the posture of the main spindle relative to the table due to variation of the tool length or projecting length of the tool cannot be corrected.